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Anonymous Poster #1

U/V, O/V, E/F Protection

04/03/2021 10:05 AM

Can i use relay (under voltage, over voltage and earth fault) to drop the power contactor as a means of protection?

What could be the problems?

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#1

Re: U/V, O/V, E/F protection

04/03/2021 10:42 AM

This site is not a permission-giving entity.

if in doubt consult an Electrical Engineer locally.

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#2
In reply to #1

Re: U/V, O/V, E/F protection

04/03/2021 11:20 AM

Thank you.

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#3

Re: U/V, O/V, E/F protection

04/03/2021 12:07 PM

Protection of what from what and where?

These are fundamental factors an electrical engineer needs to know to attempt to answer your question.

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#4

Re: U/V, O/V, E/F protection

04/03/2021 12:09 PM

The contact points could weld together creating an always on or partially on situation...it could be used as a primary layer in a layered protection scenario though...so yes you could in certain situations where the relay/contactor combination is not the only protection...

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#5

Re: U/V, O/V, E/F protection

04/03/2021 1:54 PM

I have an old (1965) book on electro-mechanical motor controls. Diagrams show limit switches, timers, float switches, pressure switches, relays, etc. in various configurations to energize and de-energize motor contactors. Since your relay(s) is/are going to be used to drop the contactor in over/under voltage/earth fault conditions, a critical circuit design question is: Is there any hazard in automatic start-up of the equipment when proper voltage is restored, or fault conditions removed? If so, then the circuit must be designed such that the motor contactor drops out and can only be re-energized via manual control.

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#6

Re: U/V, O/V, E/F protection

04/04/2021 6:31 AM

This is a standard configuration used in the MCC to control motors especially when the control is carried out by PLC initiated from a DCS..

For larger motors often a smaller contactor is interposed to energise the main contactor closing coil.

The start relay then energises the auxiliary contactor which supplies the power to the main contactor.

The main thing to be aware of is that the contactor once tripped does no auto reclose and this is achieved by the hold in contacts being an auxiliary on the main contactor or by using non latching relays of RS flip flops or their logical equivalent in the PLC

The OV, OC and EF trip should also latch the fault indication and at the same time disable any attempt to reclose the contactor until the fault is investigated and reset. If not done the gorilla on the control panel can keep restarting the device until something blows up or the control board melts down, both unsatisfactory outcomes necessitating the gorillas reeducation!

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#7

Re: U/V, O/V, E/F Protection

04/16/2021 2:01 AM

Contactor has limited capacity for fault current interruption. It is typically rated to interrupt not more than 8 times the continuous rated current of the contactor.

If the circuit includes a back up fuse upstream of power contactor to interrupt short circuit fault current and it is sized as recommended by the power contactor manufacturer, then, I suppose you can allow the relay to drop the contactor as intended by you.

Generally speaking, power contactor is opened on operation of thermal overload protection as well as earth fault protection only. Undervoltage protection and overvoltage protection are not used for opening the motor contactor (presume the contactor is non-latched type)

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